public class MyQueue {
    static class ListNode{
        public int val;
        public ListNode prev;
        public ListNode next;

        public ListNode(int val) {
            this.val = val;
        }
    }
    public ListNode first = null;
    public ListNode last = null;
    public int usedSize;
    public void offer(int val){
        ListNode node = new ListNode(val);
        if(isEmpty()){
            first=last=node;
        }else {
            last.next = node;
            node.prev = last;
            last = node;
        }
        usedSize++;
    }
    public int poll(){
        if(isEmpty()){
            return -1;
        }else {
            int val = first.val;
            first = first.next;
            if(first != null) {
                first.prev = null;
            }
            usedSize--;
            return val;
        }
    }
    public int peek(){
        if(isEmpty()){
            return -1;
        }else {
            return first.val;
        }
    }
    public boolean isEmpty(){
        return usedSize==0;
        //return first==null;
    }
}
